1. Field of the Invention
The present invention generally relates to the art of hybrid electronic circuit structures fabricated from low-temperature-cofired-ceramic (LTCC) tape, and more specifically to cofired LTCC tape structures and fabrication methods incorporating ferromagnetic elements, drop-in components and transformers.
2. Description of the Related Art
Fabrication of multilayer electronic structures for hybrid microcircuit technology and other applications includes the "thick film process" in which individual conductor and dielectric compositions in paste form are sequentially deposited on insulating substrates and then fired, one layer of material at a time, to fabricate a thick film, multilayer circuit.
A disadvantage of the thick film process is that voids or pinholes can be formed in the thick film dielectric material during the sequential printing and firing process. Another disadvantage is that the requirement for building up many multiple thick film layers in the more complex hybrid circuits results in an expensive process due to the number of individual processing steps involved. A third disadvantage is that a mistake on an individual layer requires scrapping the entire unit.
A second approach to the fabrication of hybrid microcircuits is the cofired ceramic process. This technology utilizes dielectric material formed into sheets having alumina as a main component. Individual sheets of tape are printed with metallization and other circuit patterns, stacked on each other, laminated together at a predetermined temperature and pressure, and then fired at a desired elevated temperature at which the material fuses or sinters.
Where alumina is generally used as the insulating material, tungsten, molybdenum or molymanganese is typically used for metallization, and the part is fired to about 1,600.degree. C. in an H.sub.2 reducing atmosphere.
The undesirable high processing temperature and requisite H.sub.2 atmosphere, and more importantly the electrical performance of the refractory metals has led to the development of Low-Temperature-Cofired-Ceramic (LTCC) tape. A preferred LTCC is commercially available from the DuPont Company as Green Tape.RTM. no. 851AT. The tape contains a material formulation including a mixture of glass and ceramic fillers which sinter at about 850.degree. C., and exhibits thermal expansion similar to alumina.
The low-temperature processing permits the use of air fired resistors and precious metal thick film conductors such as gold, silver, or their alloys. In the typical high-temperature process, screen-printed resistors cannot be used and only refractory metal pastes are used as conductors.
A discussion of thick film technology, and high and low temperature cofired ceramic tape technology, is found in "DEVELOPMENT OF A LOW TEMPERATURE COFIRED MULTILAYER CERAMIC TECHNOLOGY", by William Vitriol et al, ISHM Proceedings 1983, pp. 593-598.
Ferromagnetic inks have been developed for use with the thick film process described above. These inks can be screen printed together with other paste layers onto a substrate to form cores or enhancers for inductors, magnetic shield planes, and other ferromagnetic elements. However, these inks are not usable with the LTCC process because they have a dissimilar shrinkage profile to LTCC tape. This causes warping or buckling of the LTCC tape structure during firing.
For this reason, magnetic components, including transformers, as well as non-magnetic components such as heat sinks and varistors have previously been fabricated separately and fixed to the surfaces of LTCC structures. This is disadvantageous in that the space on the surfaces of the structures is severely limited, and should be utilized for the mounting of hybrid microelectronic integrated circuit chips and interconnects.